The BTR will become the D.R. if the D.R. fails designated riders are chosen on a per segment basis.
So in this example on this Ethernet segment or two may be chosen as the designated Rada.
But on this segment between our for and our 7 of 4 may be chosen as the designated Rada on a per physical
segment basis a designated route is chosen.
So just because our fall is not a designated road on this segment does not mean that our 4 is not a
designated route on the segment every Ethan its segment requires the election and maintenance of a designated
Rada.
So from an update point of view if this link goes down or one is updating its designate Harada using
this multicast address 2 2 4 0 0 6 or 2 the designated Rada updates all of its neighbors on that segment
so are 3 or 6 or 5 and are 4.
Get the update about the change of 4 then floods that update to our 7 or 7 well then flood the update
to any neighbors that it may have not shown in this diagram.
The same with our 5 or 6 or 3 and so forth.
The update will be flooded out the topology just as in this example where all four received the update
from or two and then sends it to our 7 so just to sum up designated Rod is or D elected firstly based
on the highest priority.
The range is from 0 to 255.
The default value is 1 0 disables the ability for this rata to become a designated Rada or backup designated
wrote it.
If the properties are the same the Rada with the highest Rada I.D. will become the designated Rada.
It's important to realize that pre-emption doesn't exist with designated as so in this example it's
a priority of wrote it to a set to 10 and the priority of Rada 5 was set to 5 and then the election
took place or two would become the designated Rada however.
However two went down four out of five would become the designated Rada.
And for instance or three would become the backup designated Rada.
Let's assume that the priorities of the other parties are set to 1 but rather 3 has the highest Rada
I.D. So Rada 3 becomes the designated Rada when R2 comes back again.
It will not become a designated Rada or backup designated Rada.
In other words it's not preemptive.
Because our 5 is already the designated Rada and all 3 is the backup designated Rada or two will just
accept that fact and will become known as a D or other or draw the depending which term you prefer.
It will not try and pre-empt another election and try and become the designated Rada.
All other parties including our two will become D or others in a topology like this let's talk about
the shortest Baathist algorithm or SPF algorithm the SPF algorithm places each right at the root of
the tree and calculates the shortest path to each node using the Dykstra algorithm based on the cumulative
cost that is required to reach that destination.
So as an example a four out of one wants to get to a network behind rather two.
It's going to determine the best route based on the cost which uses a formula.
Tend to the eight divided by bandwidth.
So as an example this is a T1 link no one wants to send traffic throughout it too will it take this
T1 link or will it use the port via rod or three using a 10 meg and 10 make link.
Now rip using hub count within the traffic directly to R2 but what would always be 50 OSB if by default
uses a reference bandwidth of 10 to the 8.
You can change this and you need to when you've got gig and 10 gig links of SPF has been around for
many years and in the early days there were no speeds such as gig 10 gig and so forth.
So today if you've got gig and 10 gig links you're going to want to change this reference bandwidth.
But for now let's assume that we're using the default.
So the cost of a link is tend to the eight divided by the bandwidth in bits per second a one kilohertz
per second equals 1000 bits per second one megabits per second is one million bits per second 10 megabits
per second is 10 million beats per second.
So the cost of a 10 megabits per second link is tend to the eight divided by 10 million which gives
you a cost of 10 using the same formula for a T1 link.
So for this link between r 1 and RTD the cost extend to the 8 divided by 1 5 4 4 triple 0 which gives
you a cost of four.
So the cost using this link would be sixty for the cost if the traffic was sent via R2 would be as follows
tend to the 8 to about 10 million which is 10 but they are two lengths of 10 meg.
So the total cost is 20.
So the cost via the T1 is 64.
But the cost via the 10 meg links is 20 so 0 SPF is going to choose the link via R2 to send traffic
to all three because the cost is lower based on this formula tend to the 8 divided by bandwidth.
So the default formula used for costing is tend to the eight divided by bandwidth.
Now you can change the reference bandwidth by using this command within the OSP if process order cost
reference bandwidth and then specifying a value in megabits per second.
The default is 100.
In other words the reference bandwidth is 100 megabits per second 100 megabits per second has a cost
an SPF of 1.
If you change this to a thousand as an example a gigabit link would be seen as the reference bandwidth
or if you change it to one hundred thousand one hundred gig link would be used as a reference bandwidth.
You need to do this command on all routers where you have interfaces with bandwidth.
Greater than just Ethernet.
You can also change the cost on an interface.
So rather than 0 SPF calculating the cost of the link based on the bandwidth you can set the OSP off
cost by tapping the command IP SPF cost and then specifying a value.
It's very important that the bandwidth statements on your radar interfaces be configured correctly because
that's what OSP deems the speed of the link to be.
The GOP does the same thing.
It uses bandwidth as part of the calculation when working out the shortest route to the destination.